It is conventional to use a vectorscope including a cathode ray tube (CRT) to display in polar coordinates the phase and amplitude of a chrominance signal, the amplitude of the signal being represented by the radial coordinate and the phase relative to burst being represented by the angular coordinate. A consequence of this display format is that the light dot of the CRT remains at the same position on the CRT screen if the amplitude of the chrominance signal is zero, or below some small value, regardless of the value of the relative phase. Consequently, when the duty cycle of the display is low, or the signal is predominantly of low amplitude, the center spot is by far the brightest area of the display. This leads to problems in use, such as poor viewability and difficulty in identifying the precise location of the center dot owing to haloing, and also gives rise to the danger that the phosphor of the CRT might be damaged.